Free-flow pumps of this kind, as they are known from EP 0 081 456 A1 to the applicant of the present invention, are often used in wastewater that is contaminated in particular with solid matter. In such pumps the distance between the impeller and the pump inlet is chosen such that a free flow space is formed between the inlet and the impeller exit, the free flow space constituting a passage for a sphere of a predetermined largest sphere diameter that can possibly be pumped so as to counteract the risk of clogging due to the solid components in the pumped liquid.
In practice, however, it has often been found that particularly tissue or knit materials consisting of fibers or yarns or other solids composed of two-dimensional and flexible materials tend to accumulate at the impeller front surface and obstruct the desired unimpeded passage through the vane-free space. More specifically, a short-term or even permanent accretion of such materials has been observed in the central area of the impeller. This material accretion in front of the impeller surface causes an undesirable reduction of the pumping head and of the efficiency or leads first to a reduction of the flow rate and ultimately to total clogging of the pump.